Feature: floor division

[mdickinson]

This PR:

• implements floor division, Python-style modulus, and divmod.
• switches the implementation of % on BigFloats to use Python-style semantics for negative operands.
• renames the old mod function to fmod.
• adds new mod and floordiv functions for the new % semantics and the // operation, respectively.
• removes the checks for MPFR version >= 2.4.0; we now require version 3.0 or later.

[mdickinson]

Error from Travis on Python 2.6 is:

======================================================================
ERROR: test_floordiv (bigfloat.test.test_bigfloat.BigFloatTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/mdickinson/bigfloat/bigfloat/test/test_bigfloat.py", line 361, in test_floordiv
    z = fractions.Fraction(x) // fractions.Fraction(y)
  File "/opt/python/2.6.9/lib/python2.6/fractions.py", line 99, in __new__
    numerator = operator.index(numerator)
TypeError: 'float' object cannot be interpreted as an index

[mdickinson]

Changelog needs updating.