Added missing assertion for install and remove package

| Q               | A
| --------------- | ---
| Bug fix?        | no
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Tests pass?     | yes
| Related tickets | #20
| License         | GNU GPL v3

Added some asserts:
  * After install a package:
    - Confirm the package is represented in installed.db

  * After remove a package:
    - the filelist file is gone
    - Confirm the package is not represented in installed.db

closes #20