Fixed Wolfram|Alpha bugs.

Did this by giving all operators an appropiate toString method with good
parenthesing. Strings returned by Symja were ambigious to Wolfram|Alpha
in some cases and by building our own query strings we lose this
ambuguity.
Add checks so that wolfram|alpha only queries when it can.
remove unneccecery toString method in MathObjectEmpty
Merge remote-tracking branch 'arian/wa-fix' into work

Conflicts:
	src/org/teaminfty/math_dragon/MainActivity.java
	src/org/teaminfty/math_dragon/view/math/MathConstant.java
	src/org/teaminfty/math_dragon/view/math/MathObject.java
	src/org/teaminfty/math_dragon/view/math/MathOperationRoot.java
	src/org/teaminfty/math_dragon/view/math/MathVariable.java

* Unnecessary outer parentheses are now removed from the WA query

Signed-off-by: Mark Kamsma <mark@markkamsma.nl>