ENH: port PyRadarMet `beam_block_frac` function

ENH: port PyRadarMet `half_power_radius` function

MIN: Forgot to remove the import statement for importlib.

Also removed pytz from the requirements file.

MIN: Removing dependency pytz.

pytz was only used to retrieve e tzinfo subclass for UTC. Replaced this by
our own tzinfo subclass for UTC in wradlib.util.UTC.

Also, made it more clear in comments that we parse the timestamp from the file
header, not from the file name.

MIN: Removing dependency importlib.

Replaced importlib.import_module by __import__ in util.import_optional.

This resolves issue #72.

FIX: Fixing paths in gis_export_example.

Merged wradlib/wradlib into default

FIX: change string conversion to netcdf-function, use netcdf function for iteration within varaible section

FIX: Actually use the fmt keyword argument in to_AAIGrid.

DOC: Fixing some links in the documentation of the new functions.

FIX: Fixed a top-left corner issue with the RADOLAN grid in the GeoTIFF example.

Plus some changes in the related docs.

ENH: Added a function to export cartesian grids as GeoTIFF files.

ENH: Added a function to export cartesian grids as ESRI ASCII grids.

The function writes a text file to ``fpath`` that contains the header info and
the grid data passed with the argument ``data``. For details on ESRI grids
(or Arc/Info ASCII grids) see e.g. http://en.wikipedia.org/wiki/Esri_grid....

FIX: Removed last item ("authority") from RADOLAN WTK string in create_osr.

This addresses issue #70. The item 'AUTHORITY["EPSG","1000001"]' should read
'AUTHORITY["USER","1000001"]'. However, it should rather be removed in order
to avoid conflicts with other user-defined CRS IDs.

REF: Seperation of requirements into mandatory, devel and optional.

This way, user installation via pip or setup.py will be easier.

Requirements in the ``requirements.txt`` should be only those which are needed
to run wradlib. ``requirements_optional.txt`` are optional dependencies needed to...

FIX: Flush stdout in setup.

This should address Issue #67 (pip install hangs if dependencies are missing).
Users should now see the information printed by setup.py on stdout.

I also changed the output a little that informs the user about what setup.py found.

FIX: update `release_notes.rst` and `version` after bugfix merge from release/0.5 branch

Added tag v0.5.1 for changeset 987b3c617abe

--HG--
branch : release/0.5

FIX: update `release_notes.rst` and `version` after bugfix merge from release/0.4 branch

--HG--
branch : release/0.5

Added tag v0.4.2 for changeset 01a396cb8593

--HG--
branch : release/0.4