Clean up build instructions.

1. New people should know exactly what to do, right away.

2. It should be really easy to for newbies to build & run Pash.

3. The steps on Linux, Mac, and Windows should be the same, if possible.

3. Duplication between the README, CONTRIBUTING, .travis.yml, build scripts, etc. is asking for trouble. I'll screw them up. So DRY.
Change syntax for running xbuild in travis.

I'm really just guessing here.
`xbuild /t:test,run` to use mono off-Windows
.travis.yml doesn't allow TABs
use `xterm -e` for `xbuild /t:run`
Make `PashConsole` the default startup project.

When you open a solution for the first time, Visual Studio picks the first
project in the solution as the startup project.

I hope the same is true in MonoDevelop/XamarinStudio.
Remove PashGui.

It doesn't really work, no one is working on it, and it creates a
dependency on GTK#.

Easy to add back when we are ready for it.
Fix warnings as errors in MonoDevelop

It seems MonoDevelop is unable to properly process
Condition attributes in project files.  As a result the
Conditional TreatWarningAsErrors added in SHA: e06750c cause
MonoDevelop to always set TreatWarningAsErrors to true.

This commit switches the order of the lines as MonoDevelop seems
to process the first instance and ignore the second, changing the
behaviour back to TreatWarningsAsErrors=false
Remove `Pash.settings.targets`

MonoDevelop and xbuild both have bugs around warnings and warn-as-error
that are making development a pain. Importing settings in this unorthodox
way just makes it even harder to get things working on all platforms at
the same time.
Enhance CONTRIBUTING.md

- Simplify list of development tools.

- Command line w/ xbuild.

- Command line to run tests.
Typo.
Give up on `xbuild /t:run` for now.

Running an interactive command line tool within xbuild/msbuild doesn't work well.

On Windows, we can use `start /wait`.

On XWindows we can use `xterm -e`.

I don't know what works on Mac.

I don't know what works in SSH or at Linux text terminal.

For now, let people run it themselves. Not that hard.
Merge branch 'build-steps' of https://github.com/JayBazuzi/Pash2 into build-steps
Merge pull request #34 from JayBazuzi/build-steps

Cleaner newbie experience