Add burst search functions

This PR only creates new files in `search` folder. No other modifications.

To import the burst search function:

    from search import burstsearch_py

or for the cython version:

    from search import burstsearch_c

The Cython function needs to be compiled with:

    python setup.py build_ext --inplace

Tests for the functions are in `test_bsearch.py`. To run the test execute `py.test`.