[tools] Rewritten mergejs.py and generate_api.py scripts.

[tools] Rewritten mergejs.py and generate_api.py scripts.

+ tools/generate_api.py has been rewritten to be more readable.
+ tools/mergejs.py has been rewritten to work more like C++ include directive,\
to be more of a in-place copy-paste mechanism.

Previously the only way to use //=require() was to create a file in which no code
other than the require() directives was allowed to be. If there was some code it
was removed during the preprocessing phase. After this change it is possible to
mix code and require() commands in the same file.

[Verification] I've tested this change by running TCTs for all modules that use require() directive
(all modules with js/ subdir).

tct-filesystem-tizen-tests 100% pass
tct-calendar-tizen-tests 100% pass
tct-contact-tizen-tests 100% pass
tct-content-tizen-tests 100% pass

Signed-off-by: Michal Michalski <m.michalski2@partner.samsung.com>
Change-Id: I71436797b78a1746b9ea105bc466a75d42d26ddb