Trivial formatting fixes in docs
Use unique labels for Sphinx references
Fix error raised while converting from a single unito to one expressed as the relation between many

fixes #29
Added fix to Issue #29 to Changes
Support to wrap functions and make them pint-aware

Code, test cases and documentation.

closes #16
Added issue 16 to CHANGES
Support for @import directive

The @import directive allows importing a definition file inside another.
The syntax is similar to @import in css.

    # Same folder
    @import constants_en.txt
    # Parent folder
    @import ../constants_en.txt
    # Absolute path
    @import /path/to/constants_en.txt

Semantics:
- The definitions in the imported filed are added to the registry
  when the @import statement is found
- Path are written in unix style

In addition, the default file is is provided to load_definitions using
pkg_resources.resource_filename instead of pkg_resources.resource_string
to allow for relative import of constants.

closes #22
Added constants list
Add __dir__ method to UnitRegistry. Autocompletion iPython.

closes #30
Implemented html formater with code
Implemented IPython rich display methods for latex and html.

closes #31
Updated CHANGES
Added fix to Issue #29 to Changes
Preparing release 0.2.1
Back to development: 0.3
Adjusted doc examples to Py3
Changed setup.py encoding to make 'python setup.py check -r' work in Python 2.X
Changed docs Makefile to use local sphinx-build when present
Changed formatting codes.

Reasons behind the change:
- uppercase codes are easier to read (in particular 'l' vs 'L', in certain fonts).
- remove the exclamation mark as it does not belong to the format code.
- agreed on formatting with the uncertainties package (See Issue #6).

WARNING: This is a backwards incompatible change affecting string formatting operations.

To update your code, remove any exclamation mark '!' from you
the part after the colon ':' in your format spec.
Additionally, change the letters to uppercase:
- Latex : change 'l' to 'L'
- Pretty: change 'p' to 'P'
- HTML:   change 'h' to 'H'
Merge branch 'master' into develop