Only import `contracts.useful_contracts.numpy_specific` if numpy is available.

Duplicated the style of the try/except/else block used in `src/contracts/library/__init__.py`
to conditionally load modules with numpy dependencies.
Merge pull request #16 from ryanheimbuch-wf/useful_contracts_numpy_check

Allow `contracts` to be imported when numpy is not installed.
removed make warning
1.6.0 -- 2013-09-10
-------------------

* New features:
  - New ContractsMeta metaclass to provide subclasses with contracts.
    Use like you would ABCMeta. (docs to write)
  - Added a "isinstance(ClassName)" contract for generic type checking

* Other fixes:
  - Cleaned up much of the code regarding Numpy arrays
  - Fixed a few incompatibilities with new versions of pyparsing
  - Better display of error conditions in several occasions.

* Incomplete list of contributors: Ryan Heimbuch
more docs
docs
docs
added documentation for ContractsMeta
some other examples
docs additions and reorganization
better documentation
fixed typos
demo of custom conditions in docstrings