Fix web_wallet build instructions for qt_wallet

NPM dependencies should be installed within the web_wallet directory and
not the qt_wallet directory. Must change to web_wallet directory before
executing NPM install commands.

I didn't have to set paths since I used the prebuilt dependency package
which came with QT and setenv set up the QT paths for me already.
Add cd command as code decleration
Merge pull request #43 from sidhujag/master

Update qt_wallet build instructions