Add Mac Appbundle generation support.

pkg/mac/README.txt

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+Building pgAdmin4.dmg on Mac OS X
+=================================
+
+Required Packages (Either build the sources or get them from macports or similar):
+
+1. Python installation
+  - Python 2.6 or above from https://www.python.org/
+
+2. QT installation
+  - Qt 4 or 5 from http://www.qt.io/
+
+3. PostgreSQL installation
+  - PostgreSQL 9.1 or above from http://www.postgresql.org/
+
+Building:
+
+1. Set the PYTHON_HOME environment variable to the Python root installation directory, e.g.
+
+   export PYTHON_HOME=/System/Library/Frameworks/Python.framework/Versions/2.7
+
+2. Set the QTDIR environment variable to the QT root installation directory, e.g.
+
+   export QTDIR=~/Qt/5.5/clang_64
+
+3. Set the PGDIR environment variable to the PostgreSQL installation directory, e.g.
+
+   export PGDIR=/usr/local/pgsql
+
+4. To build, go to pgAdmin4 source root directory and execute "make appbundle". This will
+   create the python virtual environment and install all the required python modules mentioned in the
+   requirements file using pip, build the runtime code and finally create the app bundle and the DMG 
+   in ./dist directory