Remove manage.py startup script from project directory.

[warownia1]

Having manage.py in the project directory causes issues when the script is loaded from outside the project directory. Additionally, the script does nothing beyond calling slivka.command.manager() which can be set-up as a slivka entry point making manage.py redundant.

• remove manage.py script entirely and create slivka or slivka-start entry point in its place
• do not use SLIVKA_SETTINGS environment variable as the script needs to be called in the project directory where settings.yml is located anyway
• allow users set SLIVKA_HOME instead where all config files are located or default to cwd. Also, remove BASE_DIR from the settings.yml as it's redundant and confusing. (BASE_DIR is always the current directory where settings.yml sits)

[warownia1]

Issue already resolved. slivka executable is created when Slivka is installed and runs slivka.cli.main entry point. The manage.py script is still present in the project directory, but it's role is reduced to setting SLIVKA_HOME variable and running slivka.cli.main() only.