Support branching instructions

[mzarnowski]

Goals:
Branching instruction consist of:
a) condition
b) target

supported conditions are:

ALWAYS, 
IF_EQUAL, IF_NOT_EQUAL, 
IF_SMALLER, IF_NOT_SMALLER,
IF_GREATER, IF_NOT_GREATER, 

IF_ZERO, IF_NOT_ZERO, 
IF_SMALLER_THAN_ZERO, IF_NOT_SMALLER_THAN_ZERO,
IF_GREATER_THAN_ZERO, IF_NOT_GREATER_THAN_ZERO, 

target should be represented by a new Label type. Alongside Label, a meta-instruction DefineLabel should be introduced. It would be interpreted during JVM instruction generation as 'assign current byte offset with given label'