Port to MacOS X (Fixes #93)

Installer script creates self-contained Trelby.app package,
which should be dragged to /Applications.  To run directly
from source without packaging, you will need to install lxml,
Python 2.7, and wxPython 1.9 to your Mac.  Then just symlink
/Applications/Trelby.app to the source directory and it should run.

The Makefile command "make dist-osx" uses py2app, which must be
installed if you want to create a self-contained package.  Once the
package is built, the result can be found in /dist.  Trelby.app
can be dragged to /Applications and should run even on Macs with older
Python or without lxml or wxPython previously installed.

If you want to install to /Applications automatically, just use:

make install-osx

This will also create a folder on the desktop with the license,
sample documents, and some other helpful files, including a simlink
to the app in the /Applications folder.

Thanks to anilgulecha and Max F for getting this port started at
http://tinyurl.com/8z8nmbb

Features:

*  Runs on MacOS X (tested on 10.6.x/10.8.x)
*  Runs as native app at /Applications/Trelby.app
*  Self-contained- no additional libraries/python required
*  Moved menu items to better match Mac conventions
*  Recognizes native Quit instruction
*  Modify "ALT"->"CMD" in shortcut settings
*  Launches Viewer.app for print feature
*  Makes the manual xslt xml->html conversion compatible with OS X
*  Adds MacOS X native warnings for missing/old Python/wx (for devs)
*  MacOS native warning to run in /Applications (for users)
*  Sets Desktop as default script destination (also fixes for unix)
*  Uses MacOS X native color palette
*  Adds installation scripts to generate standalone Trelby.app + archive
*  Adds MacOS X build instructions
*  Uses 256x256 Trelby icon as app & document icons
*  Auto-opens Trelby document on double-click from Finder

This is a first pass at this port, so there may be bugs and things
to work out.
Associate Trelby file type for Linux (Fixes #265)

Creates "application/trelby" MIME type on *nix-based
systems following freedesktop.org standards.  This
results in documents ending in ".trelby" having a
custom icon and auto-opening w/double-click.

Commit summary:

* Sets up "application/trelby" MIME type
* Any file ending in ".trelby" is associated
* Sets up five icon sizes
* Automate install by generated .deb packages
* Auto-check for xdg-* support
* If Trelby is uninstalled, filetype association
  should be automatically removed

Note that this is installed for Debian-type installations.
Just use a simple "make deb" and then dpkg -i the .deb file.

Support can be easily added to support local "make install"
This was not implemented here as the Makefile is still
installing to /opt and it is unclear if this is the final
directory or if there are plans to use debian-style
locations.  In any event, the Makefile to support /opt/trelby
is already written and can be added in another commit.

This is a first pass, but it seems to work fine.  However,
there always may be issues that need addressing.