Added basic Expression classes

A lot of details, declarations and implementations are missing because
of unresolved discussions (statements vs. assignment expressions,
primitive operations vs. macros, etc.) as well as VariableIdentifier not
holding a ref to Value for function calls.
FunctionExpression Branch
Expression && LiteralExpression
Modified tests to remove FacadeExpression
Expression and LiteralExpression

- Moved FunctionInvocationExpression and pals to another branch, no
longer in this commit
- Changed some of the tests to use LiteralExpression
- Changed Value->TypeValue in TypeMismatchException
Formatting
Removed doc blocks
Reverted TypeMismatchException
Merge pull request #39 from maxqchen/master

Added basic Expression classes
Added convenience NamespaceIdentifier constructor
Merge pull request #46 from lucaswoj/namespace-string

Added convenience NamespaceIdentifier constructor
add tests for exception classes
rebase against correct version of Expression class
Merge branch 'exception-issues' of git@github.com:MurphEngineer/whdl.git into exception-issues

Conflicts:
	src/test/java/org/whdl/frontend/syntaxtree/TestMultipleAssignmentException.java
	src/test/java/org/whdl/frontend/syntaxtree/TestVariableNotAssignedException.java