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

Fixes #360