As @vforchi said:

> The ANSI standard `||` is supported only by Oracle and Postgres: MySQL uses
> `CONCAT` and SQLServer uses `+`.

_This commit resolves the issue #70 ._

In the resulting SQL query, if there are an alias on the joined tables, these
aliases must be used in the ON clause (instead of the full table name).

For instance, the following ADQL query:

```sql
  SELECT *
  FROM tableA AS a NATURAL JOIN tableB AS b;
```

should be translated into the following SQL:

```sql
  SELECT *
  FROM tableA AS a
    INNER JOIN tableB AS b
      ON a.id = b.id
```

This commit complete the resolution of the Pull Request #16
(more details about the issue can be got in there)

In MS-SQLServer, the following functions return a value of the same
type as the given argument:
  - abs
  - ceiling
  - degrees
  - floor
  - radians
  - round.

Since ADQL requires that these functions return a DOUBLE
(i.e. a float in MS-SQLServer), it must be ensured that the given
argument is a float.

This commit partially fixes the Pull Request #16

[ADQL] SqlServerTranslator: bug fix for TOP vs LIMITgrammar. Adding support for math functions, including translations for TRUNCATE and MOD non-standard implementation.

deals with NATURAL JOINs and JOINs using the keyword USING so that being
supported by SQL Server. Basically, they are translated as a list of ON
conditions.
Warning: This translator is just guaranteed to solve the NATURAL and USING
issue. Support for datatypes conversion and case sensitivity has to be
reviewed. Besides no geometrical function is translated for SQL Server.