ENH: Add h5py version and remove backend info

This information has been removed from the string displayed in:

    $ biom show-install-info

The information now looks like this:

    System information
   ==================
   Platform: darwin
   Python/GCC version:   2.7.6 (v2.7.6:3a1db0d2747e, Nov 10 2013,
           00:42:54)  [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)]
   Python executable:
   /Users/yoshikivazquezbaeza/.virtualenvs/biom-dev/bin/python

    Dependency versions
   ===================
   pyqi version:   0.3.1
   NumPy version:  1.7.1
   SciPy version:  0.13.3
   h5py version:  2.3.0

    biom-format package information
   ===============================
   biom-format version:   1.3.1-dev

In case h5py is not installed, it will look like this:

    System information
   ==================
   Platform: darwin
   Python/GCC version:   2.7.6 (v2.7.6:3a1db0d2747e, Nov 10 2013,
           00:42:54)  [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)]
   Python executable:
   /Users/yoshikivazquezbaeza/.virtualenvs/biom-dev/bin/python

    Dependency versions
   ===================
   pyqi version:   0.3.1
   NumPy version:  1.7.1
   SciPy version:  0.13.3
   h5py version: WARNING: Not installed - this is an optional dependency.
   It is strongly recommended for large datasets.

    biom-format package information
   ===============================
   biom-format version:   1.3.1-dev