Provide support for variable substitution in epf files

Please provied support for variable substitution in epf files.

What steps will reproduce the problem?
1. Create a epf file for an eclipse property
2. Add a variable to it (e.g. ${user_homedir} for
/instance/org.eclipse.m2e.core/eclipse.m2.userSettingsFile=${user_homedir}\\.m2\
\settings.xml)
3. Point workspacemechanic to that epf
4. Update eclipse

What is the expected output? What do you see instead?
variable ${user_homedir} should be substituted, but is not.

What version of the product are you using? On what operating system?
0.3.3, Win 7, Eclipse Juno, Java 7

A solution was described in 
https://groups.google.com/forum/?fromgroups=#!topic/workspacemechanic/aECwERC160
g, but never found the way into workspacemechanic

Original issue reported on code.google.com by sebidiet...@gmail.com on 17 Sep 2012 at 9:05

Merged into: #92

demonrain / workspacemechanic

Provide support for variable substitution in epf files #99